Think of a number and write it down (7)
I believe the answer is:
compose
'down' is the definition.
(thesaurus)
'think of a number and write it' is the wordplay.
I cannot quite understand how this works, but
'of' could be 'o' and 'o' is located in the answer.
'number' could be 'm' (M is an example) and 'm' is present in the answer.
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for compose that I've seen before include "What Ireland used to do" , "Some cop (anag)" , "Write a piece of music" , "Make up, create" , "Create or put together" .)